Incandescible cathode and method of making the same



March 23 1954 H. J. LEMMENs ET A1. 2,673,277

INCANDESCIBLE CATHODE AND METHOD OF' MAKING THE SAME Filed Oct. 2l, 1950 fis.

INVENTORS HENDRIKUS J. LEMMENS 7 AGENT `tion of refractory material.

Patented Mar. 23, i954 UNITED vSTATES PATENT OFFICE INCANDESCIBLE CATHODE AND METHOD OF MAKING THE SAME Conn., as trustee Application October 21, 1950, Serial No. 191,341

Claims priority, application Netherlands October 25, 1949 3 Claims.

- 'I'his invention pertains to incandescible cathodes and to methods of making the same.

The invention is more particularly concerned with incandescible cathodes of the type in which the electron-emissive material is provided within the cavity of a body having a porous wall p0r- In such cathodes the porous wall portion is made of sintered refractory material such as tungsten, molybdenum, tantalum, hafnium, or niobium and the pores of this body form the largest apertures for the egress of the electron emissive material.

In making such cathodes considerable difiiculty has been encountered in obtaining a satisfactory seal between the porous wall portion and the remainder of the cathode. Because the pores 0f the porous body must form the largest apertures for the egress of the electron emissive material and these apertures are extremely small, it is essential that the seal between' the porous Wall portion and the remainder of the cathode body must be very tight. On the other hand since this seal must be made while the electron emissive material is in the cavity, care must be taken that the electron emissive material is not destroyed by heat.

In view of the above it was thought that seals utilizing heat could not be used satisfactorily in such constructions and it was proposed to make a mechanical seal, for instance by means of a force fit or spring material over the edge of the porous wall portion. Such mechanical seals however, were not entirely satisfactory because they had tofbe made with great care in order toensure a tight joint. In addition such seals were frequently affected by the expansion and contraction of the parts during the operation of the cathodes. As a result such mechanical seals were not well `adapted for mass production.

In accordance with the present invention the above-mentioned disadvantages of mechanical seals in such cathode structures have been overcome and, contrary to opinion that heat could not be used in effecting the seal, a satisfactory seal is obtained by welding, the porous wall portion to the body by a butt-Welding method.

This butt-welding method consists in placing the porous wall portion against the body in such a way that they contact along a rim and form a cavity for the emissive material. Then a pressure is exerted so that both pieces are in good electrical contact all along the rim and a heavy electric current of short duration is passed through this contact. Preferably a condenser discharge is used for generating the current.

In accordance with the method of the present invention the cathode body is provided with the electron emissive material and the porous wall portion of refractory material is put in place with its surface abutting the adjacent surface portion of the cathode body which is of refractory material such as molybdenum, tantalum or niobium.

In order that the invention may be more clearly understood and readily carried into effect it will now be described more fully with reference to the accompanying drawing, in which, Figures 1 to 11, inclusive, are sectional elevational views of a composite cathode structure embodying the present invention, whereas Fig. 12 gives an arrangement for the welding process.

Referring to Figure 1, numeral I designates a small cylindrical cap of a porous refractory material such as tungsten butt-welded to a hollow refractory metal member 3 at the circular edge 5 to form a cavity containing emissive material for example alkaline earth metal compounds.

A heating coil 4 is positioned in the hollow member 3.

Figure 2 shows a structure similar to that of Fig. l in which the cavity containing the emissive compounds 2 is formed by a fiat place 6 of a porous refractory material, butt-welded at edge E to the extending peripheral portion 'I of a cylindrical refractory metal member 23 having a cup shaped surface 24. The cup shaped nature of the body 23 facilitates assembly of the cathode for the butt-welding operation.

Figure 3 is a modification of Figure 2 and in which the cupshaped body 8 of molybdenum of other refractory material containing emissive compounds 2 and mounted on a hollow refractory metal member 9 is sealed by a fiat porous plate member 6 welded thereto at edge 5. In this structure a large area of surface contact between bodies 8 and 9 facilitates the transmission of heat to the emissive compound 2 from the coil d in the body 9.

Figure 4 shows an embodiment of the invention in which a cavity containing emissive compounds 2 is formed by an outer cylindrical body l0 of porous refractory material andan inner cylindrical refractory metal member ll having centric cylinders I3 of porous refractory material and I4 of refractory metal, the inner cylinder I4 having end portions I5 expanded in the direction of the outer cylinder I3 and welded thereto at outer edges 5. In this case the heating coil is positioned along the axis of the concentric cylinders I4 and passes through the ends thereof.

The cathode in Figure 6 is similar to that of Figure 5 the seal of the cavity however being affected in this structure by ring members I6 butt welded to the concentriccylinders I3 and I4 at edges 5 and 25.

The cathode structure of Figure 7 diiers from that of Figure 1 in that the body I of porous refractory material is elongated.

In Figure 8 the cylinder I3 of porous refractory material surrounding the emissive material 2 has butt welded to each of its ends a hollow refractory metal member 3 enclosing a heating coil 4. Such a structure assists in the achievement of uniform -heating and is particularly advantageous in cases where the cylinder I3 is required Ato be elongated.

The cathode 'body of Figure 9 has improved uniform-'heating characteristics over that of the structure of Figure 7 achieved by a pin-like extension of the refractory metal member I'I. The extension I8 is positioned in the body I in direct contact with the emissive material 2.

gIn Figure 10 a cylindrical capl I of porous refractory material contains a supply of emissive material 2 insulated from a heating coil 4 by a coating of material on the coil. The cap I is sealed by a plate I9 of refractory material butt welded to the cap at edge 5. The capv I9 is provided with apertures through which the leads of the heating coil 4 pass in electrically insulating relation being tightly sealed to the plate I9- at the edges of the apertures by an insulating material such as thorium oxide.

The cathode of Figure 11 is adapted for direct heating of the body I3 of porous refractory material which surrounds the emissive material 2 and is sealed by refractory metal plates 2| butt Welded thereto at the edges 5. 'Ihe plates 2| are provided with supply wires 22 for the passage of a heating current.

Each of the structures set out above is particularly adapted for mass production by the method of invention as set out hereinafter.

In Fig. 12 the cathode parts 3 and 1 are clamped against each other by means of the arms 23 and 21 of the welding apparatus. In the cavity formed by` the depression 24 the emissive material 2 is present. The arm 26 extends within the skirt 23 and on the upper arm 21 a pressure of -about 20 kilograms is exerted when thediameter of 6 is about 3 mm.

The upper arm 27 is hinged in an insulating block 28 against which arm 26 is secured.

`A welding transformer 29 is connected with a secondary winding 30 to arms 2G and 21. The primary winding 3| may be connected to a battery 32 of condensers by switch 33 which in the other position connects the battery 32 to a rectifier 34. The battery 32 of about 1000 pf. is charged to about 550 volts. In this way a satisfactory tight seal between the porous Wall body B and the 4 cathode body 1 is obtained. The term buttwelded join as employed in the claims is delined `as a joint between two metal member formed by bringing the metal members into abutting relationship to establish physical contact between the ends of the metal members, applying longitudinal pressure to both metal members and passing Van electrical current through the two members to fuse the ends of the members into a homogeneous metal structure.

What we claim is:

l'. A method of making a thermionic cathode comprising the steps of placing Within an open cavity of a member of refractory metal a supply of alkaline earth compound subject to deteriorationby heat, placing over the opening of the cavity a member of sintered refractory metal fhaving a porous portion, and locally heating abutting surface portions of said members by pressing said members together while passing through the connection between the membersY an electrical current of such intensity and duration as to form a fused joint without deleteriously affecting the alkaline earth compounds.

2. A method of making a thermionic cathode comprising the steps of placing within an open cavity of a member of refractory metal a supply of alkaline earth compounds subject to deterioration by heat, placing over the opening of the cavity a sintered tungsten member having a porous portion, and locally heating abutting surface portions of said members by pressing said members together while passing through the' connection between the members a condenser current discharge of such intensity and duration as to form a fused joint Without deleteriously affecting the alkaline earth compounds.

3. A method of making a thermionic cathode comprising the step of placing within an open cavity of a member of refractory metal a supply of alkaline earth compounds subject to deterioration by heat, placing over the opening of the cavity a sintered tungsten member having a porous portion, and locally heating abutting surface portions of said members by pressing said members together with a pressure of about 20 kgms. while passing through the connection between the members a condenser discharge of such intensity and duration as to form a fused joint without deleteriously aecting the alkaline earth compounds.

HENDRIKUS JOHANNES LEMMENS. MARINUS J ACOBUS JANSEN. J OSEPHUS ARNOLDUS BERNARDUS ZAAT.

References Cited in the le of this patent..

UNITED STATESv PATENTS Number Name Date 2.011,121 Sherman et al Aug. 13, 1935 2,021,119 Tinkham Nov. 12, 1935 2,121,589 Espe June 21, 1938 2,131,204 Waldschmidt Sept. 27, 1938 2,354,947 Colwell Aug. 1, 1944 2,375,808 Miller May 15, 1945 2,492,619 Casellini et al. Dec. 2 7, 1949 2,506,747 Smith May 9, 1950 2,543,728 Lemmens et al. Feb. 27, 1951 

1. A METHOD OF MAKING A THERMIONIC CATHODE COMPRISING THE STEPS OF PLACING WITHIN AN OPEN CAVITY OF A MEMBER OF REFRACTORY METAL A SUPPLY OF ALKALINE EARTH COMPOUND SUBJECT TO DETERIORATION BY HEAT, PLACING OVER THE OPENING OF THE CAVITY OF MEMBER OF SINTERED REFRACTORY METAL HAVING A POROUS PORTION, AND LOCALLY HEATING ABUTTING SURFACE PORTIONS OF SAID MEMBERS BY PRESSING SAID MEMBERS TOGETHER WHILE PASSING THROUGH THE CONNECTION BETWEEN THE MEMBERS AN ELECTRICAL CURRENT OF SUCH INTENSITY AND DURATION AS TO FORM A FUSED JOINT WITHOUT DELETERIOUSLY AFFECTING THE ALKALINE EARTH COMPOUNDS. 